The invention is directed to an opto-electronic image sensor arrangement. Thin-film photodiodes are arranged on a substrate in lines and are connected to a read-out circuit arranged on the same substrate via thin-film interconnects that differ in length and proceed parallel to one another. Charge carriers generated in the photodiodes by the light reflected from the image master can be read out as electrical signals.
Modern communications and office automation require read equipment that can pick up not only texts but also graphics and images as well as from a sheet of paper and can read them into a computer.
For this purpose, work has been carried out for some time in developing opto-electronic line sensors for image pick-up through the use of which such masters can be read without optical demagnification. Such line sensors can be manufactured in a hybrid technology wherein thin-film photodiodes are arranged in a row and are connected to read-out chips via thin-film interconnects. Further details regarding the structure and the properties of an image sensor on the basis of amorphous, hydrogenated silicon (a-Si:H) may be derived from a report by K. Kempter in the Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 617 Amorphous Semiconductors for Microelectronics (1986), pages 120 through 126, incorporated herein.
The light reflected by the master generates charge carriers in the photodiodes. In the integration method, the charge carriers are collected in storage capacitors over a certain time (integration time), and are read out with the readout circuit at the end of this integration time. FIG. 1 shows an equivalant circuit diagram that contains the capacitors essential for the signal formation. The storage capacitor for every photodiode is formed by the photodiode capacitance (C.sub.d) itself, by the capacitance of the corresponding input of the read-out circuit, and by the capacitances of the read-out line (C.sub.p and C.sub.c). Given a prescribed signal charge, the signal voltage at the photodiodes is inversely proportional to the size of the storage capacitance. Since the capacitances of the interconnects are part of the storage capacitances, different lengths of the read-out lines lead to corresponding inhomogeneities in the sensor signal. The light sensitivity of the sensor, the cross-talk between neighboring elements, and the modulation transmission (MTF) associated therewith are then topically dependent. Greater details regarding the influence of the interconnect capacitances on the sensor signal may be derived from the report by Rosan et al. in the Conference Volume of IEEE Conf. on Photo Electronic Imaging, London, 1985, No. 253, pages 92 through 95, incorporated herein.
There is thus the problem in the prior art of signal inhomogeneities caused by differing lengths of read-out lines. This problem has not been satisfactorily resolved in the prior art. The path of the interconnects is largely defined by the dimensions of the read-out circuit preferably integrated on a chip, by the pad arrangement thereof, and by the number of photodiodes per mm. The differing length of the read-out lines necessarily derives from the lower density of the bond pads on the read-out chips compared to the photodiodes, as may be derived from FIG. 1 in the report by Rosan and Brunst in the Conference Volume of the 1986 Materials Research Society Symposium Proceedings, Vol. 70, pages 683 through 688, incorporated herein.
In European Patent application 0 148 620 incorporated herein, this problem is dealt with for an image sensor wherein a ground plane (grounded metallic shielding layer) entirely or partially covers the interconnects, and is separated from the interconnects by an insulator layer. The interconect and ground plane then forms a type of plate capacitor. A stray capacitance thereby arises between every interconnect and the ground plane, this stray capacitance being higher the longer the line. In order to compensate the signal inhomogeneities resulting therefrom, the sum of all stray capacitances connected to a cell must be made of the same size in this case. For this purpose, it is proposed to make the area of all interconnects identical in size, or to provide the capacitance balancing via an additional overlapped capacitance that is not illuminated and is connected to the cell.